The LED manufacturing process typically involves fabrication of arrayed LED units on a printed circuit board (PCB) substrate. The fabrication process includes die attachment, wire bonding, phosphor application and lens attachment, followed by singulation to separate the PCB into the individual LED units. The singulated LED units are tested individually and sorted into bins according to optical and electrical performance before packing.
Optical testing of an LED unit is typically performed by positioning the LED within the input port of an integrating sphere which is coupled to a detector (e.g., to a spectrometer via an optical fiber), measuring optical parameters of the LED unit, and calibrating the LED unit by comparing the measured optical parameters against those of a reference LED on which absolute calibration has previously been performed.
A disadvantage of this post-singulation unit-by-unit testing process is the amount of handling required for each LED, which increases the complexity and cost of the testing, as well as the risk of contamination or damage of the LED.
One previously proposed approach is to directly test the LEDs on the substrate, i.e. prior to singulation, for example using a BTS256-LED tester of Gigahertz-Optik GmbH (Puchheim, Germany). A problem with this approach is that in addition to direct detection of light emitted by the device under test (DUT), an integrating sphere used as part of the testing process can detect indirect light from inactive LEDs adjacent to the DUT. Light from the DUT is reflected from the integrating sphere, is absorbed by phosphors of the adjacent LEDs, and is in turn re-emitted into the detector aperture. In order to alleviate this problem the BTS256-LED, which is a hand-held tester, includes a conical adapter at the input of the integrating sphere, the conical adapter being positionable over the DUT to block out any re-emitted light. However, this device is generally not suitable for high-throughput testing applications as it requires precise manual positioning in both the horizontal and vertical directions in order for the conical adapter to be effective.
There remains a need for a simpler, more cost-effective and higher yield process flow for the testing of LEDs.